Re: [RFC PATCH v7 1/7] Restartable sequences system call

[Mathieu Desnoyers]

----- On Aug 10, 2016, at 3:50 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:

> On Tue, Aug 09, 2016 at 10:41:47PM +0000, Mathieu Desnoyers wrote:
>> #ifdef __LP64__
>> 
>> static bool rseq_update_cpu_id_event_counter(struct task_struct *t)
>> {
>>         union rseq_cpu_event u;
>> 
>>         u.e.cpu_id = raw_smp_processor_id();
>>         u.e.event_counter = ++t->rseq_event_counter;
>>         if (__put_user(u.v, &t->rseq->u.v))
>>                 return false;
>>         trace_rseq_inc(t->rseq_event_counter);
>>         return true;
>> }
>> 
>> #else /* #ifdef __LP64__ */
>> 
>> static bool rseq_update_cpu_id_event_counter(struct task_struct *t)
>> {
>>         if (__put_user(raw_smp_processor_id(), &t->rseq->u.e.cpu_id))
>>                 return false;
>>         if (__put_user(++t->rseq_event_counter, &t->rseq->u.e.event_counter))
>>                 return false;
>>         trace_rseq_inc(t->rseq_event_counter);
>>         return true;
>> }
>> 
>> #endif /* #else #ifdef __LP64__ */
> 
> I don't think you need to guard it (and CONFIG_64BIT is the 'right'
> kernel symbol for that), 32bit should have u64 __put_user() only
> implemented as 2 u32 stores.

OK, I can then simplify the implementation to:

[...]
 * On 64-bit architectures, both cpu_id and event_counter can be updated
 * with a single 64-bit store. On 32-bit architectures, __put_user() is
 * expected to perform two 32-bit single-copy stores to guarantee
 * single-copy atomicity semantics for other threads.
 */
static bool rseq_update_cpu_id_event_counter(struct task_struct *t)
{
        union rseq_cpu_event u;

        u.e.cpu_id = raw_smp_processor_id();
        u.e.event_counter = ++t->rseq_event_counter;
        if (__put_user(u.v, &t->rseq->u.v))
                return false;
        trace_rseq_inc(t->rseq_event_counter);
        return true;
}

Thanks!

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com